Cooling-equipment working-fluid composition

ABSTRACT

The present invention provides a working fluid composition for a refrigerating machine, comprising: a refrigerating machine oil comprising, as a base oil, a mixed ester of (A) a complex ester and (B) a polyol ester, wherein (A) is obtainable by synthesis from a specific polyhydric alcohol, a polybasic acid having 6 to 12 carbon atoms, and a monohydric alcohol having 4 to 18 carbon atoms or a monocarboxylic acid having 4 to 18 carbon atoms, wherein (B) is obtainable by synthesis from a specific polyhydric alcohol and a monocarboxylic acid having 4 to 18 carbon atoms, and wherein a mass ratio of (A)/(B) is 5/95 to 95/5; and difluoromethane used as a refrigerant, and the working fluid composition having a refrigerant dissolved viscosity of 5.5 mm 2 /s or more, at a temperature of 80° C. and an absolute pressure of 2.1 MPa.

TECHNICAL FIELD

The present invention relates to a working fluid composition for arefrigerating machine. It is noted that the term “refrigerating machine”herein includes a vehicle air conditioner, a dehumidifier, arefrigerator, a refrigerated warehouse, a vending machine, a showcase, acooling apparatus used in a chemical plant or the like, a household airconditioner, a package air conditioner, a heat pump for hot watersupply, and the like.

BACKGROUND ART

In the field of refrigeration and air conditioning,1,1,1,2-tetrafluoroethane (R134a) that is a hydrofluorocarbon (HFC) andR410A that is a mixed refrigerant of difluoromethane (R32) andpentafluoroethane (R125) in a mass ratio of 1/1 are currently widelyused as refrigerants for a refrigerator, a vehicle air conditioner, aroom air conditioner, industrial refrigerating machine, and the like.Although such HFC refrigerants have an ozone depletion potential (ODP)of zero, they have a high global warming potential (GWP) of 1000 ormore, and therefore, their usage is limited in accordance with what iscalled the F-gas regulations aiming at the global environmentalprotection.

As a substitute for a refrigerant having a high GWP, single use ofdifluoromethane (R32) is under examination as a candidate owing to itsthermodynamic characteristic. Although the GWP of R32 is 675 and ratherhigh, it is under examination as a potential candidate because its gaspressure is high and it is a highly efficient refrigerant.

In using a refrigerant increasing a pressure, such as R32 or a mixedrefrigerant containing R32, since the discharge temperature in acompressor becomes high, an oil film of a refrigerating machine oil forattaining lubrication in the compressor becomes thin to cause a severelubrication condition, and therefore, a refrigerating machine oil havinggood lubricity is necessary.

Patent Literature 1 proposes, as a refrigerating machine oil for an R32refrigerant, an ester synthesized from trimethylolpropane and/orneopentyl glycol, a specific dibasic acid, and a monohydric alcohol or amonocarboxylic acid, and Patent Literature 2 proposes, as one for an R32single refrigerant or a mixed refrigerant containing 50% or more of R32,an ester-based refrigerating machine oil that contains 20 to 60% by massof a monobasic fatty acid having 5 or less carbon atoms, uses a branchedfatty acid as a fatty acid having 8 or more carbon atoms, and has aviscosity at 40° C. of 32 to 100 cst. With respect to a base oil for ageneral industrial lubricating oil, Patent Literature 3 proposes alubricating oil base oil containing a synthetic ester obtained byreacting an alcohol component containing 90% by mass or more oftrimethylolpropane with a carboxylic acid component that contains amonocarboxylic acid having 8 to 12 carbon atoms and adipic acid, andfurther contains, in a total amount of 90% by mass or more, caprylicacid and/or a monocarboxylic acid having 8 to 12 carbon atoms containing90% by mass or more of caprylic acid in total, and adipic acid.

CITATION LIST Patent Literature

Patent Literature 1: Japanese Patent Application Laid-Open No.2011-184536

Patent Literature 2: Japanese Patent Application Laid-Open No.2002-129179

Patent Literature 3: Japanese Patent Application Laid-Open No.2012-102235

SUMMARY OF INVENTION Technical Problem

There is a trend, however, that a load condition in a sliding portion ofa refrigerating machine becomes more severe in the future, andtherefore, there is a demand for a working fluid showing an excellentantiwear property in a case where an R32 refrigerant coexists and isdissolved in a refrigerating machine oil.

The present invention was accomplished in consideration of theabove-described problem, and an object is to provide a working fluidcomposition for a refrigerating machine that can retain a thick oil filmand show a high antiwear effect and is good in long-term reliabilityeven under a severe lubrication condition caused when a refrigerantcoexists and is dissolved in a refrigerating machine oil.

Solution to Problem

The present inventors have discovered that a working fluid forrefrigeration/air conditioning, which comprises a refrigerating machineoil using, as a base oil, an ester containing a complex ester obtainableby synthesis from specific polyhydric alcohol, polybasic acid,monohydric alcohol or monocarboxylic acid, and a polyol ester obtainableby synthesis from specific polyhydric alcohol and monocarboxylic acid;and difluoromethane used as a refrigerant, and has a high refrigerantdissolved viscosity under a specific condition, forms a thick oil filmto show high antiwear property, resulting in accomplishing the presentinvention.

Specifically, the present invention provides a working fluid compositionfor a refrigerating machine according to the following [1] to [6]:

[1] A working fluid composition for a refrigerating machine, comprising:a refrigerating machine oil comprising, as a base oil, a mixed ester of(A) a complex ester and (B) a polyol ester, wherein (A) the complexester is obtainable by synthesis from at least one polyhydric alcoholselected from neopentyl glycol, trimethylolpropane and pentaerythritol,a polybasic acid having 6 to 12 carbon atoms, and a monohydric alcoholhaving 4 to 18 carbon atoms or a monocarboxylic acid having 4 to 18carbon atoms, wherein (B) the polyol ester is obtainable by synthesisfrom at least one polyhydric alcohol selected from neopentyl glycol,trimethylolpropane, pentaerythritol and dipentaerythritol, and amonocarboxylic acid having 4 to 18 carbon atoms, and wherein a massratio of (A) the complex ester/(B) the polyol ester is 5/95 to 95/5; anddifluoromethane used as a refrigerant, and the working fluid compositionhaving a refrigerant dissolved viscosity of 5.5 mm²/s or more, at atemperature of 80° C. and an absolute pressure of 2.1 MPa.[2] The working fluid composition for a refrigerating machine accordingto [1] above, wherein the polyhydric alcohol constituting (A) thecomplex ester is neopentyl glycol and/or trimethylolpropane.[3] The working fluid composition for a refrigerating machine accordingto [1] or [2] above, wherein the polybasic acid constituting (A) thecomplex ester is adipic acid and/or sebacic acid.[4] The working fluid composition for a refrigerating machine accordingto any one of [1] to [3] above, wherein the monohydric alcoholconstituting (A) the complex ester is an alcohol having 8 to 10 carbonatoms.[5] The working fluid composition for a refrigerating machine accordingto any one of [1] to [4], wherein (B) the polyol ester is an esterobtainable by synthesis from neopentyl glycol and/or pentaerythritol anda monocarboxylic acid having 4 to 9 carbon atoms.[6] The working fluid composition for a refrigerating machine accordingto any one of [1] to [5] above, wherein (B) the polyol ester is an esterobtainable by synthesis from pentaerythritol and a mixed acid of abranched fatty acid having 4 carbon atoms and 3,5,5-trimethylhexanoicacid.

Advantageous Effects of Invention

A working fluid composition for a refrigerating machine of the presentinvention has, even under a severe lubrication condition in which theviscosity of a refrigerating machine oil is lowered because adifluoromethane refrigerant is dissolved therein, high antiwear propertyand excellent stability, and exhibits a remarkable effect that anapparatus can be used stably over a long period of time.

DESCRIPTION OF EMBODIMENTS

A preferred embodiment of the present invention will now be described indetail.

A working fluid composition for a refrigerating machine according to theembodiment of the present invention comprises: a refrigerating machineoil containing, as a base oil, a mixed ester of (A) a complex ester and(B) a polyol ester, in which (A) the complex ester is obtainable bysynthesis from at least one polyhydric alcohol selected from neopentylglycol, trimethylolpropane and pentaerythritol, a polybasic acid having6 to 12 carbon atoms, and a monohydric alcohol having 4 to 18 carbonatoms or a monocarboxylic acid having 4 to 18 carbon atoms, (B) thepolyol ester is obtainable by synthesis from at least one polyhydricalcohol selected from neopentyl glycol, trimethylolpropane,pentaerythritol and dipentaerythritol, and a monocarboxylic acid having4 to 18 carbon atoms, and a mass ratio of (A) the complex ester/(B) thepolyol ester is 5/95 to 95/5; and difluoromethane used as a refrigerant,in which a refrigerant dissolved viscosity of the working fluidcomposition, at a temperature of 80° C. and an absolute pressure of 2.1MPa, is 5.5 mm²/s or more.

The aforementioned complex ester has a lower compatibility with arefrigerant because the refrigerant is difficult to dissolve therein ascompared with a conventional refrigerating machine oil, but has acharacteristic that it can retain an oil film thick. Besides, theaforementioned polyol ester is good in the compatibility with arefrigerant. In the present embodiment, since the complex ester and thepolyol ester thus having different characteristics (of, particularly, arefrigerant solubility) are blended, the oil film can be made thick incoexistence of a refrigerant, and accordingly, the antiwear property ofa resultant working fluid can be improved.

Incidentally, a complex ester is difficult to be compatible with arefrigerant because it has a high molecular weight and hence is an esterwith a high viscosity, and it is not suitably used singly as a base oilof a refrigerating machine oil requiring compatibility with arefrigerant from the viewpoint of oil return to a compressor. As onecharacteristic of the present embodiment, it is possible to make thecharacteristics balanced by mixing the complex ester with an oil havinga good compatibility with a refrigerant, such as the above-describedpolyol ester.

A preferable kinematic viscosity of the complex ester is 20 to 500 mm²/sat 4° C., a more preferable kinematic viscosity is 40 to 400 mm²/s, afurther preferable kinematic viscosity is 50 to 200 mm²/s, and aparticularly preferable kinematic viscosity is 50 to 90 mm²/s. Besides,the viscosity index is preferably 100 or more, and particularlypreferably 110 to 160.

Examples of a synthesizing method for the complex ester include (A) amethod in which a molar ratio between a polyhydric alcohol and apolybasic acid is adjusted for obtaining an ester intermediate having acarboxyl group of the polybasic acid remaining therein, and the carboxylgroup is esterified by a monohydric alcohol, and (B) a method in which amolar ratio between a polyhydric alcohol and a polybasic acid isadjusted for obtaining an ester intermediate having a hydroxyl group ofthe polyhydric alcohol remaining therein, and the hydroxyl group isesterified by a monocarboxylic acid. A complex ester obtained by themethod of (B) above is rather inferior in stability to a complex esterobtained by the method of (A) above because a comparatively strong acidis produced if the former is hydrolyzed when used as a refrigeratingmachine oil. The complex ester of the present embodiment is preferably acomplex ester with higher stability obtained by the method of (A) above.

The polyhydric alcohol constituting the complex ester is preferablyneopentyl glycol or trimethylolpropane for attaining a suitableviscosity as a base oil. Incidentally, if tetravalent pentaerythritol isused, as compared with a case of using neopentyl glycol ortrimethylolpropane, the viscosity of a resultant complex ester tends tobe higher and a low temperature characteristic also tends to be poorer.Besides, neopentyl glycol whose viscosity can be widely adjusted is morepreferred.

Besides, the polyhydric alcohol constituting the complex esterpreferably further contains, in addition to at least one selected fromneopentyl glycol, trimethylolpropane and pentaerythritol, a dihydricalcohol having 2 to 10 carbon atoms except neopentyl glycol because thusthe lubricity can be improved. Examples of the dihydric alcohol having 2to 10 carbon atoms except neopentyl glycol include ethylene glycol,propanediol, butanediol, pentanediol, hexanediol,2-methyl-1,3-propanediol, 3-methyl-1,5-pentanediol and2,2-diethyl-1,3-pentanediol, and butanediol that can attain goodcharacteristic balance of a resultant synthesized base oil is preferred,and examples of the butanediol include 1,2-butanediol, 1,3-butanediol,1,4-butanediol and 2,3-butanediol, among which 1,3-butanediol and1,4-butandiol are more preferred from the viewpoint of characteristics.The dihydric alcohol having 2 to 10 carbon atoms except neopentyl glycolis used in an amount of preferably 1.2 mol or less, particularlypreferably 0.8 mol or less, and further preferably 0.4 mol or less basedon 1 mol of the polyhydric alcohol selected from neopentyl glycol,trimethylolpropane and pentaerythritol.

The polybasic acid constituting the complex ester is a polybasic acidhaving 6 to 12 carbon atoms. Examples of such a polybasic acid includeadipic acid, pimelic acid, suberic acid, azelaic acid, sebacic acid,phthalic acid and trimellitic acid, among which adipic acid and sebacicacid that are good in the characteristic balance of a resultantsynthesized ester and in availability are preferred, and in particular,adipic acid is more preferred. The polybasic acid is used in an amountof preferably 0.4 to 4 mol, particularly preferably 0.5 mol to 3 mol,and further preferably 0.6 mol to 2.5 mol based on 1 mol of thepolyhydric alcohol selected from neopentyl glycol, trimethylolpropaneand pentaerythritol.

If a carboxyl group remains in a complex ester intermediate producedthrough the reaction between the polyhydric alcohol and the polybasicacid, the carboxyl group is esterified by a monohydric alcohol having 4to 18 carbon atoms. Examples of the monohydric alcohol having 4 to 18carbon atoms include straight or branched butanol, straight or branchedpentanol, straight or branched hexanol, straight or branched heptanol,straight or branched octanol, straight or branched nonanol, straight orbranched decanol, straight or branched dodecanol, and an aliphaticalcohol such as oleyl alcohol. From the viewpoint of the characteristicbalance, monohydric alcohols having 6 to 10 carbon atoms, and 8 to 10carbon atoms in particular are preferred, among which 2-ethylhexanol and3,5,5-trimethylhexanol are preferred from the viewpoint of a good lowtemperature characteristic of the synthesized complex ester.

Alternatively, if a hydroxyl group remains in the complex esterintermediate produced through the reaction between the polyhydricalcohol and the polybasic acid, the hydroxyl group is esterified by amonocarboxylic acid having 4 to 18 carbon atoms. Examples of themonocarboxylic acid having 4 to 18 carbon atoms include straight orbranched butanoic acid, straight or branched pentanoic acid, straight orbranched hexanoic acid, straight or branched heptanoic acid, straight orbranched octanoic acid, straight or branched nonanoic acid, straight orbranched decanoic acid, straight or branched dodecanoic acid, and oleicacid. Preferably, monocarboxylic acids having 8 to 10 carbon atoms areused, among which 2-ethylhexanoic acid and 3,5,5-trimethylhexanoic acidare preferred from the viewpoint of the low temperature characteristic.

On the other hand, the polyol ester of the present embodiment is apolyol ester obtainable by synthesis from at least one polyhydricalcohol selected from neopentyl glycol, trimethylolpropane,pentaerythritol and dipentaerythritol, and a monocarboxylic acid having4 to 18 carbon atoms.

As the polyhydric alcohol constituting the polyol estertrimethylolpropane and pentaerythritol are preferred, andpentaerythritol is more preferred from the viewpoint of thecharacteristic balance.

Examples of the monocarboxylic acid having 4 to 18 carbon atomsconstituting the polyol ester include straight or branched butanoicacid, straight or branched pentanoic acid, straight or branched hexanoicacid, straight or branched heptanoic acid, straight or branched octanoicacid, straight or branched nonanoic acid, straight or branched decanoicacid, straight or branched dodecanoic acid, and oleic acid. From theviewpoint of the low temperature characteristic, a monocarboxylic acidhaving 4 to 9 carbon atoms is preferred, and branched butanoic acid,branched pentanoic acid, branched hexanoic acid, branched heptanoicacid, 2-ethylhexanoic acid and 3,5,5-trimethylhexanoic acid are morepreferred.

From the viewpoint of the compatibility with a refrigerant, a polyolester obtainable by synthesis from a pentaerythritol used as thepolyhydric alcohol and a mixed acid of branched fatty acid having 4 to 9carbon atoms used as the monocarboxylic acid is the most preferred.

In the working fluid composition for a refrigerating machine of thepresent embodiment, assuming that the content of the complex ester is(A) and the content of the polyol ester is (B), the ratio (A)/(B) in amass ratio is 5/95 to 95/5, and preferably 20/80 to 80/20, and morepreferably 30/70 to 70/30 in order to take more advantages of thecharacteristics of the respective esters.

The kinematic viscosity at 40° C. of the refrigerating machine oil ofthe present embodiment is preferably 3 to 500 mm²/s, more preferably 8to 150 mm²/s, and further more preferably 20 to 100 mm²/s. Besides, theviscosity index of the refrigerating machine oil is preferably 50 ormore and particularly preferably 80 to 120.

The pour point of the refrigerating machine oil of the presentembodiment is preferably −10° C. or less, and more preferably −20° C. orless.

The acid value of the refrigerating machine oil of the presentembodiment can be preferably 0.1 mgKOH/g or less, and more preferably0.05 mgKOH/g or less in order to prevent corrosion of a metal used inrefrigerating machine or piping and to suppress degradation of therefrigerating machine oil itself. It is noted that the acid value hereinmeans an acid value measured in accordance with JIS K2501 “determinationmethod of acid value”.

The flash point of the refrigerating machine oil of the presentembodiment is preferably 120° C. or more, and more preferably 200° C. ormore.

The moisture content of the refrigerating machine oil of the presentembodiment is preferably 200 ppm or less, more preferably 100 ppm orless, and most preferably 50 ppm or less. In particular, if it is usedin hermetic refrigerating machine, the moisture content is required tobe small from the viewpoint of the stability and electric insulation ofthe refrigerating machine oil.

Besides, the refrigerating machine oil of the present embodiment mayfurther contain, in addition to the complex ester and the polyol esterdescribed above, another base oil such as a mineral base oil or asynthetic base oil. The total content of the above complex ester and thepolyol ester is preferably 80% by mass or more, and particularlypreferably 95% by mass or more of the refrigerating machine oil.

With respect to refrigerating machine, there is a trend thatconventionally used HFC refrigerants having a high GWP are shifted torefrigerants having a low GWP from the viewpoint of the prevention ofglobal warming as described above, and therefore, refrigerating machineoils suitable for such refrigerants are necessary, and a suitableworking fluid of a mixture of a refrigerant and a refrigerating machineoil is demanded. According to the present invention, difluoromethane iscontained as the refrigerant, and the content of the difluoromethane inthe refrigerant is preferably 60 to 100% by mass, and particularlypreferably 80 to 100% by mass.

The mixing ratio between the refrigerating machine oil composition andthe refrigerant in the working fluid composition for a refrigeratingmachine of the present embodiment is not especially limited, and theamount of the refrigerating machine oil composition is preferably 1 to500 parts by mass, and more preferably 2 to 40 parts by mass based on100 parts by mass of the refrigerant.

The refrigerant dissolved viscosity, at a temperature of 80° C. and anabsolute pressure of 2.1 MPa, of the working fluid composition for arefrigerating machine of the present embodiment is 5.5 mm²/s or more,and preferably 6.0 mm²/s or more. Besides, the refrigerant dissolvedviscosity, at a temperature of 80° C. and an absolute pressure of 2.1MPa, of the working fluid composition for a refrigerating machine of thepresent embodiment can be 10.0 mm²/s or less.

The working fluid composition of the present embodiment can furthercontain various additives for further improving the antiwear property. Apreferable example of the additives includes a phosphate, andparticularly preferable compounds are triphenyl phosphate (TPP) andtricresyl phosphate (TCP).

Besides, suitable examples of a sulfur additive include sulfides, andthere are a large number of sulfide compounds, among which a monosulfidecompound is preferred. This is because, for example, a highly activesulfur compound such as a disulfide compound degrades the stability of arefrigerating machine oil and changes the quality of copper often usedwithin refrigerating machine.

The working fluid composition for a refrigerating machine of the presentembodiment can contain, in addition to the aforementioned additives,additives conventionally used in a lubricating oil, such as anantioxidant, a friction modifier, an antiwear agent, an extreme pressureagent, a rust inhibitor, a metal deactivator and an antifoaming agent,in a range not impairing the object of the present invention, forfurther improving the performance.

As the antioxidant, a phenol-based compound such asdi-tert-butyl-p-cresol, an amine-based compound such asalkyldiphenylamine, or the like can be contained. In particular, it ispreferable to contain a phenol-based compound antioxidant in an amountof 0.02 to 0.5% by mass based on the total amount of the refrigeratingmachine oil.

Examples of the friction modifier include aliphatic amines, aliphaticamides, aliphatic imides, alcohols, esters, phosphate amine salts andphosphite amine salts, an example of the antiwear agent includes zincdialkyidithiophosphate, examples of the extreme pressure agent includeolefin sulfide and sulfurized fats and oils, examples of the rustinhibitor include alkenyl succinic esters or partial esters, examples ofthe metal deactivator include benzotriazole and benzotriazolederivatives, and examples of the antifoaming agent include siliconecompounds and polyester compounds.

EXAMPLES

The present invention will now be described more specifically on thebasis of Examples and Comparative Examples, and it is noted that thepresent invention is not limited to the following Examples at all.

Examples 1 to 12 and Comparative Examples 1 to 5

In Examples 1 to 12 and Comparative Examples 1 to 5, base oils havingcompositions shown in Tables 1 to 3 were first prepared by using thefollowing base materials.

[A] Complex Esters

(A-1) An ester (having a kinematic viscosity at 40° C. of 68.8 mm²/s anda viscosity index of 120) obtained by causing an ester intermediateresulting from a reaction between trimethylolpropane (1 mol) and adipicacid (2.4 mol) to further react with 2-ethylhexanol (2.0 mol) anddistilling off a remaining unreacted substance.

(A-2) An ester (having a kinematic viscosity at 40° C. of 71.5 mm²/s anda viscosity index of 114) obtained by causing an ester intermediateresulting from a reaction between neopentyl glycol (1 mol) and adipicacid (0.8 mol) to further react with 3,5,5-trimethylhexanoic acid (0.5mol) and distilling off a remaining unreacted substance.

(A-3) An ester (having a kinematic viscosity at 40° C. of 77.3 mm²/s anda viscosity index of 148) obtained by causing an ester intermediateresulting from a reaction of trimethylolpropane (1 mol) and1,3-butanediol (0.2 mol) with sebacic acid (2.4 mol) to further reactwith normal heptanol (1.6 mol) and distilling off a remaining unreactedsubstance.

(A-4) An ester (having a kinematic viscosity at 40° C. of 68.2 mm²/s anda viscosity index of 144) obtained by causing an ester intermediateresulting from a reaction of neopentyl glycol (1 mol) and 1,4-butanediol(0.3 mol) with adipic acid (2.4 mol) to further react with2-ethylhexanol (2.4 mol) and distilling off a remaining unreactedsubstance.

(A-5) An ester (having a kinematic viscosity at 40° C. of 32.2 mm²/s anda viscosity index of 161) obtained by causing an ester intermediateresulting from a reaction of neopentyl glycol (1 mol) and 1,4-butanediol(0.4 mol) with adipic acid (3.1 mol) to further react with3,5,5-trimethylhexanol (3.5 mol) and distilling off a remainingunreacted substance.

(A-6) An ester (having a kinematic viscosity at 40° C. of 67.8 mm²/s anda viscosity index of 145) obtained by causing an ester intermediateresulting from a reaction of neopentyl glycol (1 mol) and 1,4-butanediol(0.3 mol) with adipic acid (2.4 mol) to further react with3,5,5-trimethylhexanol (2.5 mol) and distilling off a remainingunreacted substance.

(A-7) An ester (having a kinematic viscosity at 40° C. of 150.0 mm²/sand a viscosity index of 138) obtained by causing an ester intermediateresulting from a reaction of neopentyl glycol (1 mol) and 1,4-butanediol(0.2 mol) with adipic acid (1.9 mol) to further react with3,5,5-trimethylhexanol (1.0 mol) and distilling off a remainingunreacted substance.

[B] Polyol Esters

(B-1) An ester (having a kinematic viscosity at 40° C. of 8.3 mm²/s anda viscosity index of 56) of neopentyl glycol and 2-ethylhexanoic acid.

(B-2) An ester (having a kinematic viscosity at 40° C. of 69.4 mm²/s anda viscosity index of 95) of pentaerythritol and a mixed acid of 2-methylpropanoic acid and 3,5,5-trimethylhexanoic acid in a molar ratio of35:65.

(B-3) An ester (having a kinematic viscosity at 40° C. of 46.0 mm²/s anda viscosity index of 78) of pentaerythritol and a mixed acid of 2-methylpropanoic acid and 3,5,5-trimethylhexanoic acid in a molar ratio of60:40.

(B-4) An ester (having a kinematic viscosity at 40° C. of 70.1 mm²/s anda viscosity index of 90) of pentaerythritol and a mixed acid of2-ethylhexanoic acid and 3,5,5-trimethylhexanoic acid in a molar ratioof 45:55.

(B-5) An ester (having a kinematic viscosity at 40° C. of 23.4 mm²/s anda viscosity index of 118) of pentaerythritol and a mixed acid of normalpentanoic acid and 3,5,5-trimethylhexanoic acid in a molar ratio of80:20.

(B-6) An ester (having a kinematic viscosity at 40° C. of 51.0 mm²/s anda viscosity index of 105) of pentaerythritol and a mixed acid of normalpentanoic acid and 3,5,5-trimethylhexanoic acid in a molar ratio of40:60.

In these base oils, synthesis reactions of the esters of [A] and [B] areperformed without using a catalyst and a solvent, and a slight amount ofimpurities was removed by an adsorption treatment (a clay treatment) inthe final process. Incidentally, the kinematic viscosity and theviscosity index were measured and calculated in accordance with JISK2283.

Next, a refrigerating machine oil was prepared by mixingdi-tert-butyl-p-cresol (DBPC) serving as an antioxidant in an amount of0.1% by mass with each of the base oils of Examples 1 to 12 andComparative Examples 1 to 5.

The refrigerating machine oils of Examples 1 to 12 and ComparativeExamples 1 to 5 were subjected to measurement of dissolved viscosity anda lubricity test as follows.

(Measurement of Refrigerant Dissolved Viscosity)

A 200 ml pressure resistant vessel in which a vibration type viscometerwas put was charged with 100 g of each of the refrigerating machineoils, and after vacuum degassing the vessel, a working fluid compositionwas prepared by adding an R32 refrigerant thereto, and the viscosity wasmeasured with the pressure of the refrigerant and the temperature of thepressure resistant vessel adjusted to conditions of a temperature of 80°C. and an absolute pressure of 2.1 MPa. The obtained results are shownin Tables 1 to 3.

(Lubricity Test)

For the lubricity test, a high pressure ambience friction and weartester (employing a rotating/sliding method using a rotating vane memberand a fixed disk member) manufactured by Shinko Engineering Co., Ltd.capable of attaining a refrigerant atmosphere similar to that of anactual compressor was used. The test conditions were: oil amount: 600ml, test temperature: 110° C., rotation speed: 500 rpm, applied load: 80kgf, test time: 1 hour, and SKH-51 and FC250 were used respectively asthe vane member and the disk member. As the test conditions, R32 wasused as the refrigerant, and the inside pressure of a test vessel wasset to 3.1 MPa. The evaluation of the antiwear property was made on thebasis of a wear depth of the vane member because the wear amount of thedisk member is extremely small. The obtained results are shown in Tables1 to 3.

TABLE 1 Exam- Exam- Exam- Exam- Exam- ple 1 ple 2 ple 3 ple 4 ple 5Example 6 Composition of base oil (mass %) A-1 — — 30 — — — A-2 — — — 80— — A-3 — 40 — — — 60 A-4 — — — — — — A-5 — — — — — — A-6 50 — — — 40 —A-7 — — — — — — B-1 — — — — — — B-2 50 60 70 20 — — B-3 — — — — — — B-4— — — — 60 40 B-5 — — — — — — B-6 — — — — — — Kinematic   68.6   72.4  69.2   71.1   69.2   74.3 viscosity at 40° C. (mm²/s) Refrigerant  6.5   6.3   6.1   6.4   6.6   6.8 dissolved viscosity (mm²/s)Lubricity   7.2   7.8   8.4   8.5   8.1   7.0 test: Wear depth of vane(μm)

TABLE 2 Exam- Exam- Exam- Exam- Exam- Exam- ple 7 ple 8 ple 9 ple 10 ple11 ple 12 Composition of base oil (mass %) A-1 60 — — — — — A-2 — 70 — —— — A-3 — — — — — — A-4 — — — — — 40 A-5 — — — — — — A-6 — — — — 20 —A-7 — — 30 15 — — B-1 40 30 — — — — B-2 — — — — — 60 B-3 — — 70 — — —B-4 — — — — — — B-5 — — — 85 — — B-6 — — — — 80 — Kinematic   25.4  31.8   65.3   32.0   54.1   72.6 viscosity at 40° C. (mm²/s)Refrigerant   6.2   6.3   5.7   5.5   6.2   6.1 dissolved viscosity(mm²/s) Lubricity test:   8.4   8.6   7.2   8.3   7.3   8.3 Wear depthof vane (μm)

TABLE 3 Comp. Comp. Comp. Comp. Comp. Exam- Exam- Exam- Exam- Exam- ple1 ple 2 ple 3 ple 4 ple 5 Composition of base oil (mass %) A-1 — — — — —A-2 — — — — — A-3 — — — — — A-4 — — — — — A-5 — — — — 100 A-6 — — — — —A-7 — — — — — B-1 — — 100 — — B-2 100 — — — — B-3 — — — — — B-4 — 100 —— — B-5 — — — — — B-6 — — — 100 — Kinematic 69.4 70.1 8.3 51.0 32.2viscosity at 40° C. (mm²/s) Refrigerant 5.0 5.1 4.5 5.6 5.7 dissolvedviscosity (mm²/s) Lubricity test: 13.5 11.5 14.9 13.4 12.8 Wear depth ofvane (μm)

It is understood that the working fluid compositions of Examples 1 to 12have high refrigerant dissolved viscosities and are good in the antiwearproperty.

INDUSTRIAL APPLICABILITY

A working fluid composition for a refrigerating machine of the presentinvention has a high viscosity when a difluoromethane refrigerant isdissolved therein, and is good in antiwear property even under a severelubrication condition, and therefore exhibits a remarkable effect ofgreatly improving lubricity. Accordingly, it can be suitably used in arefrigeration/air conditioning system with high cooling efficiencyincluding a compressor, a condenser, a throttle device, an evaporatorand the like for circulating a refrigerant among these, in particular,in a system including a rotary type, swing type or scroll typecompressor, and hence is useful in the fields of room air conditioners,package air conditions, refrigerators, vehicle air conditioners,industrial refrigerating machine and the like.

The invention claimed is:
 1. A working fluid composition for arefrigerating machine, comprising: a refrigerating machine oilcomprising, as a base oil, a mixed ester of (A) a complex ester and (B)a polyol ester, wherein (A) the complex ester is obtainable by synthesisof at least one polyhydric alcohol selected from neopentyl glycol,trimethylolpropane and pentaerythritol, a polybasic acid having 6 to 12carbon atoms, and a monohydric alcohol having 4 to 18 carbon atoms or amonocarboxylic acid having 4 to 18 carbon atoms, wherein (B) the polyolester is obtainable by synthesis of at least one polyhydric alcoholselected from neopentyl glycol, trimethylolpropane, pentaerythritol anddipentaerythritol, and a monocarboxylic acid having 4 to 18 carbonatoms, and wherein a mass ratio of (A) the complex ester/(B) the polyolester is 5/95 to 80/20; and difluoromethane used as a refrigerant, andthe working fluid composition having a refrigerant dissolved viscosityof 5.5 mm²/s or more, at a temperature of 80° C. and an absolutepressure of 2.1 MPa.
 2. The working fluid composition for arefrigerating machine according to claim 1, wherein the polyhydricalcohol constituting (A) the complex ester is neopentyl glycol and/ortrimethylolpropane.
 3. The working fluid composition for a refrigeratingmachine according to claim 1, wherein the polybasic acid constituting(A) the complex ester is adipic acid and/or sebacic acid.
 4. The workingfluid composition for a refrigerating machine according to claim 1,wherein the monohydric alcohol constituting (A) the complex ester is analcohol having 8 to 10 carbon atoms.
 5. The working fluid compositionfor a refrigerating machine according to claim 1, wherein (B) the polyolester is an ester obtainable by synthesis from neopentyl glycol and/orpentaerythritol and a monocarboxylic acid having 4 to 9 carbon atoms. 6.The working fluid composition for a refrigerating machine according toclaim 1, wherein (B) the polyol ester is an ester obtainable bysynthesis from pentaerythritol and a mixed acid of a branched fatty acidhaving 4 carbon atoms and 3,5,5-trimethylhexanoic acid.